UNIT 2. INTRODUCTION TO DC GENERATOR (Part 1) OBJECTIVES. General Objective

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1 DC GENERATOR (Part 1) E2063/ Unit 2/ 1 UNIT 2 INTRODUCTION TO DC GENERATOR (Part 1) OBJECTIVES General Objective : To apply the basic principle of DC generator, construction principle and types of DC generator. Specific Objectives : At the end of the unit you will be able to: State the principle by which generators convert mechanical energy to electrical energy. Describe the construction of a generator Draw the diagram of a simple DC generator State what component causes a generator to produce direct current rather than alternating current State the classifications of DC generators State the rule to be applied when you determine the direction of induced e.m.f. in a coil. Describe types of DC generator and their characteristics Distinguish shunt, series, and compound windings of generator Explain the action of a commutator and commutation process Explain the effects of adding additional coils and poles Describe armature reaction Describe compensating windings and interpoles Explain motor reaction in a generator Calculate generated e.m.f. for a generator using E = V + I a R a

2 DC GENERATOR (Part 1) E2063/ Unit 2/ 2 INPUT 2.0 Introduction A generator is a machine that converts mechanical energy into electrical energy by using the principle of magnetic induction. This principle states that the voltage is generated in the conductor whenever a conductor is moved within a magnetic field in such a way that the conductor cuts across magnetic lines of flux. The AMOUNT of voltage generated depends on i. the strength of the magnetic field, ii. the angle at which the conductor cuts the magnetic field, iii. the speed at which the conductor is moved, and iv. the length of the conductor within the magnetic field. The POLARITY of the voltage depends on the direction of the magnetic lines of flux and the direction of movement of the conductor. To determine the direction of current in a given situation, the LEFT-HAND RULE FOR GENERATORS (Fleming s Left Hand Rule) is used. This rule is explained in the following manner. Extend the thumb, forefinger, and middle finger of your left hand at right angles to one another, as shown in figure 2.1.Point your thumb in the direction the conductor is being moved. Point your forefinger in the direction of magnetic flux (from north to south). Your middle finger will then point in the direction of current flow in an external circuit to which the voltage is applied.

3 DC GENERATOR (Part 1) E2063/ Unit 2/ 3 Figure 2.1. Fleming s left-hand rule for generators. 2.1 Types of DC Generator DC generators are classified according to the method of their field excitation. These groupings are: (i) Separately-excited generators, where the field winding is connected to a source of supply other than the armature of its own machine. (ii) Self-excited generators, where the field winding receives its supply from the armature of its own machine, and which are sub-divided into (a) shunt, (b) series and (c) compound wound generators Separately excited generator A typical separately excited generator is shown in Figure 2.2. When a load is connected across the armature terminals, a load current I a will flow. The terminal voltage V will fall from its opencircuit e.m.f. E due to a volt drop caused by current flowing through the armature resistance, shown as R a i.e. Terminal voltage, V = E I a R a Generated e.m.f., E = V + I a R a

4 DC GENERATOR (Part 1) E2063/ Unit 2/ 4 A separately-excited generator is used only in special cases, such as when a wide variation in terminal potential difference is required, or when exact control of the field current is necessary. Source: Electrical and Electrical Principles and Technology, Reprint 2001 by John Bird Characteristics Figure 2.2 Separately-excited generator The two principal generator characteristics are the generated voltage/field current characteristics, called the open-circuit characteristics and the terminal voltage/load current characteristics, called the load characteristics. A typical separately-excited generator open-circuit characteristic is shown in Figure 2.2 and a typical load characteristic is shown in Fig Source: Electrical and Electrical Principles and Technology, Reprint 2001 by John Bird Figure 2.3 Load characteristic of separately-excited generator

5 DC GENERATOR (Part 1) E2063/ Unit 2/ 5 Example 2.1 Determine the terminal voltage of a generator which develops an e.m.f. of 200 V and has an armature current of 30 A on load. Assume the armature resistance is 0.30 Ω. Solution to Example 2.1 With reference to Fig. 2-3, terminal voltage, V = E I a R a = 300 (30)(0.30) = = 191 V Shunt-wound generator In a shunt wound generator the field winding is connected parallel to the armature as shown in Figure 2.4. The field winding has a relatively high resistance and therefore the current carried is only a fraction of the armature current. Source: Electrical and Electrical Principles and Technology, Reprint 2001 by John Bird Figure 2.4 Shunt-wound generator

6 DC GENERATOR (Part 1) E2063/ Unit 2/ 6 Characteristics The generated e.m.f., E is proportional to Φ ω, at constant speed, since ω = 2πn, E Φ. Also the flux Φ is proportional to field current If until magnetic saturation of the iron circuit of the generator occurs. Hence the open circuit characteristic is as shown in Figure 2.5. Source: Electrical and Electrical Principles and Technology, Reprint 2001 by John Bird Figure 2.5 Open circuit characteristic Example 2.2 A shunt generator supplies a 20 kw load at 200 V through cables of resistance, R = 100 mω. If the field winding resistance, R f = 50Ω and the armature resistance R a = 40 mω, determine (a) the terminal voltage (b) the e.m.f. generated in the armature Solution to Example 2.2 (a) The circuit is shown in Fig. 2.6 Load current, watts I = = 100A 200 volts Volt drop in the cables to the load = IR = (100) ( ) = 10 V Hence, terminal voltage, V = = 210 Volts

7 DC GENERATOR (Part 1) E2063/ Unit 2/ 7 Source: Electrical and Electrical Principles and Technology, Reprint 2001 by John Bird Figure 2.6 (b) Armature current I a = I f + I V Field current, I f R = f 210 = = A Hence I a = I f + I = = Generated e.m.f. E = V + I a R a = (104.2)( ) = = V Series-wound generator In the series-wound generator the field winding is connected in series with the armature as shown in Figure 2.7. In series-wound generator, the field winding is in series with the armature and it is not possible to have a value of field current when the terminals are open circuited, thus it is not possible to obtain an open-circuit characteristic. Series wound generators are rarely used in practice, but can be used as a booster on DC transmission lines.

8 DC GENERATOR (Part 1) E2063/ Unit 2/ 8 Source: Electrical and Electrical Principles and Technology, Reprint 2001 by John Bird Figure 2.7 Series-wound generator Characteristics The load characteristic is the terminal voltage/current characteristic. The generated e.m.f. E, is proportional to Φ ω and at constant speed ω = 2πn is a constant. Thus E is proportional to Φ. For values of current below magnetic saturation of the yoke, poles, air gaps and armature core, the flux is proportional to the current, hence E I. For values of current above those required for magnetic saturation, the generated e.m.f. is approximately constant. The values of field resistance and armature resistance in a series wound machine are small, hence the terminal voltage V is very nearly equal to E. A typical load characteristic for a series generator is shown in Fig Source: Electrical and Electrical Principles and Technology, Reprint 2001 by John Bird Figure 2.8 Load characteristic for a series generator

9 DC GENERATOR (Part 1) E2063/ Unit 2/ Compound-wound generator In the compound-wound generator two methods of connection are used, both having a mixture of shunt and series windings, designed to combine the advantages of each, Fig. 2.9 shows a longshunt compound generator, and shows a short term compound generator. The latter is the most generally used form of DC generator. Compound-wound generators are used in electric arc welding, with lighting sets and with marine equipment. Source: Electrical and Electrical Principles and Technology, Reprint 2001 by John Bird Characteristics Figure 2.9 Compound-wound generator In cummulative compound machines the magnetic flux produced by the series and shunt fields are additive. Included in this group are over-compounded, level-compounded and undercompounded machines. The degree of compounding obtained depending on the number of turns of wire on the series winding. A typical load characteristic for a compound-wound generator is shown in Fig Source: Electrical and Electrical Principles and Technology, Reprint 2001 by John Bird Figure 2.10 Characteristic for a compound-wound generator

10 DC GENERATOR (Part 1) E2063/ Unit 2/ 10 Example 2.3 A short-shunt compound generator supplies 80 A at 200 V. If the field resistance, R f = 40 Ω, The series resistance, R se = 0.02 Ω and the armature resistance, R a = 0.04 Ω, determine the e.m.f. generated. Solution to Example 2.3 The circuit is shown in Fig Voltage drop in series winding = IR se = (80)(0.02) = 1.6 V. Potential difference across the field winding = potential difference across armature = V 1 = = V Field current I f V = R f = = 5.04 A Armature current, I a = I + I f = = A Generated e.m.f., = V 1 + I a R a = (85.04)(0.04) = = 205 V Figure 2.11

11 DC GENERATOR (Part 1) E2063/ Unit 2/ 11 Example 2.4 A 100 kw, 240 V shunt generator has a field resistance of 55Ω and armature resistance of 0.067Ω. Find the full-load generated voltage. Solution to Example 2.4 Fig 2.12 shows the shunt generator circuit. Figure 2.12 I L = = A 240 I sh = = 4.36 A 55 I a = I L + I sh = = A E g = V + I a R a = = V

12 DC GENERATOR (Part 1) E2063/ Unit 2/ 12 Activity 2A TEST YOUR UNDERSTANDING BEFORE YOU CONTINUE TO THE NEXT INPUT! 2.1 What principle is used when generators convert mechanical motion to electrical energy? 2.2 What rule should you use to determine the direction of induced e.m.f. in a coil? 2.3 Why is no e.m.f. induced in a rotating coil when it passes through the neutral plane? 2.4 A generator is connected to a 50Ω load and a current of 10 A flows. If the armature resistance is 0.5 Ω, determine (a) the terminal voltage (b) the generated e.m.f. 2.5 A short-shunt compound generator supplies 50 A at 300 V. If the field resistance is 30 Ω, the series resistance 0.03 Ω and the armature resistance 0.05Ω, determine the e.m.f. generated. 2.6 A shunt generator supplies a 50 kw load at 400 V through cables of resistance 0.2 Ω. If the field winding resistance is 50 Ω and the armature resistance is 0.05 Ω, determine (a) the terminal voltage (b) the e.m.f. generated in the armature 2.7 A long shunt compound generator has full-load output of 100 kw at 250 volts.the armature series and shunt windings have resistances of 0.05Ω, 0.03Ω and 55Ω respectively. Find the armature current and generated e.m.f.

13 DC GENERATOR (Part 1) E2063/ Unit 2/ 13 Feedback to Activity 2A 2.1 Magnetic induction. 2.2 The left-hand rule for generators. 2.3 No flux lines are cut. 2.4 (a) 500 Volts (b) 505 Volts V 2.6 (a) V (b) V A, V

14 DC GENERATOR (Part 1) E2063/ Unit 2/ 14 INPUT 2.2 DC Generator Principle of Operation Simple DC generators contain an armature (or rotor), a commutator, brushes, and field winding. The figure 2.12 below shows a simple DC or direct current generator.a variety of sources can supply mechanical energy to the DC generator to turn its armatures in order for its coils to cut through the lines of force in a magnetic field. These sources include steam, wind, a waterfall, or even an electric motor. In a direct current generator, the commutator's job is to change the alternating current (AC), which flows into its armature, into direct current. To put it another way, commutators keep the current flowing in one direction instead of back and forth. They accomplish this task by keeping the polarity of the brushes stationed on the outside of the generator positive. The commutator is made up of copper segments, with a pair (of segments) for every armature coil being insulated from all the others. The stationary brushes, which are graphite connectors on the generator, form contact with opposite parts of the commutator. As the armature coil turns, it cuts Across the magnetic field, and current is induced. At the first half turn of the armature coil (clockwise direction), the contacts between communicator and brushes are reversed. The first brush now contacts the opposite segment that it was touching during the first half turn, while the second brush contacts the segment opposite the one it touched during the first half turn. By doing this, the brushes keep current going on one direction, and deliver it to and from its destination. When a DC generator contains only a single coil, it provides a pulsating DC output. Therefore, scientists use a number of coils to produce a more stable output.

15 DC GENERATOR (Part 1) E2063/ Unit 2/ 15 Figure 2.12 Simple direct current generator The Elementary DC Generator A single-loop generator with each terminal connected to a segment of a two-segment metal ring is shown in figure The two segments of the split metal ring are insulated from each other. This forms a simple COMMUTATOR. The commutator in a DC generator replaces the slip rings of the AC generator. This is the main difference in their construction. The commutator mechanically reverses the armature loop connections to the external circuit. This occurs at the same instant that the polarity of the voltage in the armature loop reverses. Through this process the commutator changes the generated AC voltage to a pulsating DC voltage as shown in the graph of figure This action is known as commutation. For the remainder of this discussion, refer to figure 2.13, parts A through D. This will help you in following the step-by-step description of the operation of a DC generator. When the armature loop rotates clockwise from position A to position B, a voltage is induced in the armature loop which causes a current in a direction that deflects the meter to the right. Current flows through loop, out of the negative brush, through the meter and the load, and back through

16 DC GENERATOR (Part 1) E2063/ Unit 2/ 16 the positive brush to the loop. Voltage reaches its maximum value at point B on the graph for reasons explained earlier. The generated voltage and the current fall to zero at position C. At this instant each brush makes contact with both segments of the commutator. As the armature loop rotates to position D, a voltage is again induced in the loop. In this case, however, the voltage is of opposite polarity. The voltages induced in the two sides of the coil at position D are in the reverse direction to that of the voltages shown at position B. Note that the current is flowing from the black side to the white side in position B and from the white side to the black side in position D. However, because the segments of the commutator have rotated with the loop and are contacted by opposite brushes, the direction of current flow through the brushes and the meter remains the same as at position B. The voltage developed across the brushes is pulsating and unidirectional (in one direction only). It varies twice during each revolution between zero and maximum. This variation is called RIPPLE. A pulsating voltage, such as that produced in the preceding description, is unsuitable for most applications. Therefore, in practical generators more armature loops (coils) and more commutator segments are used to produce an output voltage waveform with less ripple. Figure 2.13 Effects of commutation.

17 DC GENERATOR (Part 1) E2063/ Unit 2/ 17 Activity 2B TEST YOUR UNDERSTANDING BEFORE YOU CONTINUE TO THE NEXT INPUT! 2.8 What component causes a generator to produce DC voltage rather than AC voltage at its output terminals? 2.9 At what point should brush contact change from one commutator segment to the next? 2.10 An elementary, single coil, DC generator will have an output voltage with how many pulsations per revolution?

18 DC GENERATOR (Part 1) E2063/ Unit 2/ 18 Feedback to Activity 2B 2.8 A commutator 2.9 The point at which the voltage is zero across the two segments Two

19 DC GENERATOR (Part 1) E2063/ Unit 2/ Effects of Adding Additional Coils and Poles The effects of additional coils may be illustrated by the addition of a second coil to the armature. The commutator must now be divided into four parts since there are four coil ends (see fig. 2.14).The coil is rotated in a clockwise direction from the position shown. The voltage induced in the white coil, DECREASES FOR THE NEXT 90 of rotation (from maximum to zero). The voltage induced in the black coil INCREASES from zero to maximum at the same time. Since there are four segments in the commutator, a new segment passes each brush every 90 instead of every 180. This allows the brush to switch from the white coil to the black coil at the instant the voltages in the two coils are equal. The brush remains in contact with the black coil as its induced voltage increases to maximum, level B in the graph. It then decreases to level A, 90 later. At this point, the brush will contact the white coil again. Figure 2.14 Effects of additional coils.

20 DC GENERATOR (Part 1) E2063/ Unit 2/ 20 The graph in figure 2.14 shows the ripple effect of the voltage when two armature coils are used. Since there are now four commutator segments in the commutator and only two brushes, the voltage cannot fall any lower than at point A. Therefore, the ripple is limited to the rise and fall between points A and B on the graph. By adding more armature coils, the ripple effect can be further reduced. Decreasing ripple in this way increases the effective voltage of the output. Effective voltage is the equivalent level of DC voltage, which will cause the same average current through a given resistance. By using additional armature coils, the voltage across the brushes is not allowed to fall to as low a level between peaks. Compare the graphs in figure 2.13 and Notice that the ripple has been reduced. Practical generators use many armature coils. They also use more than one pair of magnetic poles. The additional magnetic poles have the same effect on ripple as did the additional armature coils. In addition, the increased number of poles provides a stronger magnetic field (greater number of flux lines). This, in turn, allows an increase in output voltage because the coils cut more lines of flux per revolution Electromagnetic Poles Nearly all practical generators use electromagnetic poles instead of the permanent magnets used in our elementary generator. The electromagnetic field poles consist of coils of insulated copper wire wound on soft iron cores, as shown in figure The main advantages of using electromagnetic poles are (1) increased field strength and (2) a means of controlling the strength of the fields. By varying the input voltage, the field strength is varied. By varying the field strength, the output voltage of the generator can be controlled.

21 DC GENERATOR (Part 1) E2063/ Unit 2/ 21 Figure 2.15 Four-pole generator (without armature) Commutation Commutation is the process by which a DC voltage output is taken from an armature that has an AC voltage induced in it. You should remember from our discussion of the elementary DC generator that the commutator mechanically reverses the armature loop connections to the external circuit. This occurs at the same instant that the voltage polarity in the armature loop reverses. A DC voltage is applied to the load because the output connections are reversed as each commutator segment passes under a brush. The segments are insulated from each other. In figure 2.16, commutation occurs simultaneously in the two coils that are briefly shortcircuited by the brushes. Coil B is short-circuited by the negative brush. Coil Y, the opposite coil, is short-circuited by the positive brush. The brushes are positioned on the commutator so that each coil is short-circuited as it moves through its own electrical neutral plane. As you have seen previously, there is no voltage generated in the coil at that time. Therefore, no sparking can occur between the commutator and the brush. Sparking between the brushes and the commutator is an indication of improper commutation. Improper brush placement is the main cause of improper commutation.

22 DC GENERATOR (Part 1) E2063/ Unit 2/ 22 Figure 2.16 Commutation of a DC generator Armature Reaction From previous study, you know that all current-carrying conductors produce magnetic fields. The magnetic field produced by current in the armature of a DC generator affects the flux pattern and distorts the main field. This distortion causes a shift in the neutral plane, which affects commutation. This change in the neutral plane and the reaction of the magnetic field is called ARMATURE REACTION. You know that for proper commutation, the coil short-circuited by the brushes must be in the neutral plane. Consider the operation of a simple two-pole DC generator, shown in figure View A of the figure shows the field poles and the main magnetic field. The armature is shown in a simplified view in views B and C with the cross section of its coil represented as little circles. The symbols within the circles represent arrows. The dot represents the point of the arrow coming toward you, and the cross represents the tail, or feathered end, going away from you. When the armature rotates clockwise, the sides of the coil to the left will have current flowing toward you, as indicated by the dot. The side of the coil to the right will have current flowing away from you, as indicated by the cross. The field generated around each side of the

23 DC GENERATOR (Part 1) E2063/ Unit 2/ 23 coil is shown in view B of figure This field increases in strength for each wire in the armature coil, and sets up a magnetic field almost perpendicular to the main field. Figure 2.17 Armature reaction. Now you have two fields - the main field, view A, and the field around the armature coil, view B. View C of figure 2.17 shows how the armature field distorts the main field and how the neutral plane is shifted in the direction of rotation. If the brushes remain in the old neutral plane, they will be short-circuiting coils that have voltage induced in them. Consequently, there will be arcing between the brushes and commutator. To prevent arcing, the brushes must be shifted to the new neutral plane Compensating Windings and Interpoles Shifting the brushes to the advanced position (the new neutral plane) does not completely solve the problems of armature reaction. The effect of armature reaction varies with the load current. Therefore, each time the load current varies, the neutral plane shifts. This means the brush position must be changed each time the load current varies. In small generators, the effects of armature reaction are reduced by actually mechanically shifting the position of the brushes. The practice of shifting the brush position for each current variation is not practiced except in small generators. In larger generators, other means are taken

24 DC GENERATOR (Part 1) E2063/ Unit 2/ 24 to eliminate armature reaction. COMPENSATING WINDINGS or INTERPOLES are used for this purpose (fig. 2.18). The compensating windings consist of a series of coils embedded in slots in the pole faces. These coils are connected in series with the armature. The series-connected compensating windings produce a magnetic field, which varies directly with armature current. Because the compensating windings are wound to produce a field that opposes the magnetic field of the armature, they tend to cancel the effects of the armature magnetic field. The neutral plane will remain stationary and in its original position for all values of armature current. Because of this, once the brushes have been set correctly, they do not have to be moved again. Figure 2.18 Compensating windings and interpoles. Another way to reduce the effects of armature reaction is to place small auxiliary poles called "interpoles" between the main field poles. The interpoles have a few turns of large wire and are connected in series with the armature. Interpoles are wound and placed so that each interpole has the same magnetic polarity as the main pole ahead of it, in the direction of rotation. The field generated by the interpoles produces the same effect as the compensating winding. This field, in effect, cancels the armature reaction for all values of load current by causing a shift in the neutral plane opposite to the shift caused by armature reaction. The amount of shift caused by the interpoles will equal the shift caused by armature reaction since both shifts are a result of armature current.

25 DC GENERATOR (Part 1) E2063/ Unit 2/ Motor Reaction in a Generator When a generator delivers current to a load, the armature current creates a magnetic force that opposes the rotation of the armature. This is called MOTOR REACTION. A single armature conductor is represented in figure 2.18, view A. When the conductor is stationary, no voltage is generated and no current flows. Therefore, no force acts on the conductor. When the conductor is moved downward (fig. 2.18, view B) and the circuit is completed through an external load, current flows through the conductor in the direction indicated. This sets up lines of flux around the conductor in a clockwise direction. Figure 2.19 Motor reaction in a generator.

26 DC GENERATOR (Part 1) E2063/ Unit 2/ 26 KEY FACTS 1. Generator converts mechanical energy to electrical energy. 2. The generated voltage reaches its maximum value when the conductor cuts flux at the angle of The LEFT-HAND RULE FOR GENERATORS (Fleming s Left Hand Rule) can be used to determine the direction of induced current.

27 DC GENERATOR (Part 1) E2063/ Unit 2/ 27 SELF-ASSESSMENT 2 You are approaching success. Try all the questions in this self-assessment section and check your answers with those given in the Feedback on Self-Assessment 2 given on the next page. If you face any problems, discuss it with your lecturer. Good luck. Question 2-1 a. A converts mechanical energy into electrical energy. b. A converts electrical energy into mechanical energy. c. In a DC generator, the relationship between the generated voltage, terminal voltage, current and armature resistance is given by E = d. Which of the following statement is false? (i) A commutator is necessary as part of DC motor to keep the armature rotating in the same direction. (ii) The brushes of a DC machine are usually made of carbon and do not rotate with the armature. (iii)the field winding of a DC machine is housed in slots on the armature. e. State one principle application for (a) a shunt generator (b) a series generator (c) a compound generator. f. Determine the terminal voltage of a generator which develops an e.m.f. of 230 V and has an armature current of 30 A on load. Assume the armature resistance is 0.50 Ω. g. A shunt generator supplies a 40 kw load at 300 V through cables of resistance, R = 250 mω. If the field winding resistance, R f = 50Ω and the armature resistance R a = 40 mω, determine (a) the terminal voltage (b) the e.m.f. generated in the armature h. A short-shunt compound generator supplies 60 A at 200 V. If the field resistance, R f = 50 Ω, The series resistance, R se = 0.04 Ω and the armature resistance, R a = 0.04 Ω, determine the e.m.f. generated.

28 DC GENERATOR (Part 1) E2063/ Unit 2/ 28 Question 2-2 a. State any four basic parts of a DC generator? b. What does commutation achieve? c. What is the armature reaction? d. What do you understand of motor reaction in a generator?

29 DC GENERATOR (Part 1) E2063/ Unit 2/ 29 FEEDBACK TO SELF-ASSESSMENT 2 Have you tried the question????? If YES, check your answers now Answer of Question 2-1 a. generator b. motor. c. E = V + I a R a d. (c) e. (i) battery charging (ii) booster (iii) electric arc welding f. 215 V g. (i) V (ii) V h. 205 V Answer of Question 2-2 a. rectangular coil, permanent magnet, brushes, commutator. b. To take a DC voltage output from an armature that has an AC voltage induced in it c. The armature reaction is the change in neutral plane and the reaction of the magnetic field. d. When a generator delivers current to a load, the armature current creates a magnetic force that opposes the rotation of the armature.

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